This invention relates generally to reverse osmosis apparatus and, more particularly, relates to the employment of a channelized porous backing member in a reverse osmosis module to improve the efficiency and output of the module.
The present invention is particularly applicable to reverse osmosis systems used for removing or separating dissolved solids from an aqueous solution such as, for example, in desalinizing seawater. Reverse osmosis systems of this type pass seawater, under pressure, through a semipermeable membrane and recover the then desalinized water on the opposite side of said membrane. The salt crystals, having been separated from the seawater, remain on the influent side of the membrane. In order to effect desalinization in this manner, sufficient pressure must be applied to overcome the osmotic pressure of the system, which, in the case of seawater, generally ranges between 300 and 400 p.s.i.g. Commercial operation of such a system requires the passage of large volumes of water through the membrane and, accordingly, driving pressures in excess of 1000 p.s.i.g. are frequently employed.
Reverse osmosis modules heretofore used generally comprise a semipermeable membrane carried by a previous membrane support member which, in turn, contacts or is additionally supported by a backing member adapted to permit the passage of desalinized water therethrough, and furthermore to direct the water permeate to a collection means. Depending upon the system employed, a solid base, such as a metallic plate, may be added for further support.
In known reverse osmosis systems of the "spiral wound" configuration, these components (absent a metallic supporting plate) are wrapped, in spiral form, about a perforated center tube through which the desalinized water or permeate is collected for eventual distribution.
Since the semipermeable membrane is usually only about 0.0025 to about 0.001 inches thick, it is extremely fragile and thus, must be supported by a suitable membrane support structure. This membrane support structure should be of sufficient strength to support the membrane from collapsing under hydraulic pressures in excess of 1000 p.s.i.g., yet still allow the desalinized permeate to pass through to the porous backing member which not only provides further support for the delicate membrane, but serves to direct the permeate to a suitable collection means.
Due to the relatively high pressures under which reverse osmosis system must operate, it is important that the backing member component be fabricated of a material having sufficient strength to resist compacting under the high pressures, since any such compaction of this material would result in an increased back pressure reflected across the membrane interface. Further, in those instances where spiral wound systems are employed, the thickness of the backing material should ideally be less than 0.015 inches to avoid undue bulkiness of the module and consequent reduction in efficiency. Additionally, the backing material should be porous enough to readily permit the passage of water therethrough at a rate at least equal to the rate of flow of such fluid through the semipermeable membrane. In order to accomplish this at maximum efficiency, the porous backing member should be capable of carrying the permeate away from the membrane preferably in a direction normal to the direction of the fluid passing through the membrane.
Heretofore numerous backing materials have been investigated and employed, each of which, however, has been found to suffer from one drawback or another. Among those tried and rejected include materials fabricated from glass or plastic beads; screen materials constructed from nylon, aluminum, or plastic; and woven brass cloth.
More recently, woven fabrics have become increasingly popular due to the large open areas between the individual weave strands and thus the relatively high porosity of these materials. One such fabric is a woven Dacron polyester tricot. Since, as mentioned above, the tricot backing material is normally subjected to high contact pressures, it has been found that even this material undergoes a marked rearrangement of the individual fibers leading to a corresponding increase in flow resistance. To prevent this rearrangement, attempts have been made to impregnate the tricot with polymeric materials such as polystyrenes, polycarbonates, phenolics, epoxies and so on. For example, a melamine formaldehyde resin has been employed as an impregnating agent and has been found to give the tricot improved stiffness and consequently increased pressure resistance. The use of this material in reverse osmosis applications is more fully described in U.S. Pat. No. 3,813,334 as well as in the Office of Saline Water Research and Development Progress Report No. 702.
While the resin impregnated tricot is currently being commercially utilized, there are certain disadvantages inherent in the use of this backing material in systems where significant volumes of fluid are being processed. Resin impregnated tricot is relatively expensive and it has only limited water carrying capability, particularly in a direction normal to the direction of fluid passing through the membrane and the tricot. This correspondingly increases the back pressure relative to the semipermeable membrane and thus, prevents the module from operating at maximum efficiency. Further, the resin impregnated tricot fails to offer a uniformly smooth surface devoid of irregularities and, as such, may result in premature module failure due to torn or punctured membranes.
Against the foregoing background, it is a primary objective of the present invention to provide a reverse osmosis membrane module comprising an improved porous backing member.
It is another object of this invention to provide a reverse osmosis membrane module including a porous backing member exhibiting greatly improved water carrying capabilities particularly in a direction normal to the direction of the fluid passing through the module membrane.
It is an additional object of the present invention to provide a reverse osmosis membrane module, the cost of which is lower than that of modules heretofore used.
It is still another object of the invention to produce a reverse osmosis membrane module which is less subject to premature failure than modules heretofore used.